In recent years, polycarbonate resins play an important role in engineering plastics and have been developed based on their superior transparency and mechanical properties, since polycarbonate polymerization methods using bisphenol A were developed by Bayer and GE in 1950. At present, worldwide sales of polycarbonate are about 3,400,000 tones and polycarbonates are utilized in a variety of fields including optical applications including optical discs such as CDs and DVDs, extrusion sheets and films, electric and electronic products and automotive molding parts, based on their superior physical properties. Annual worldwide rate of growth in production of polycarbonates is 7% and is at least 10% in developing nations including China.
Commercialization of polycarbonate processes was initiated by Bayer in 1958 and was successively completed by GE in 1960. The initial commercialization process was a “phosgene process” in which bisphenol A and a phosgene gas are used as monomers, and the interfacial polymerization process using phosgene gas, is currently used as a polycarbonate process in various nations. However, the phosgene process is restricted due to facility safety and environmental problems resulting from use of the toxic gas and other problems such as large energy consumption and high construction costs derived from use of methylene chloride and water caused by selection of interfacial polymerization. As a result, a great deal of research had been continuously made on non-interfacial polymerization as a substitute for the interfacial polymerization, and development of a non-phosgene process has been initiated, since melt polymerization using bisphenol A and diphenyl carbonate was developed by Bayer in the early 1960s. In recent years, Asahi Kasei has successfully commercialized a process for preparing diphenyl carbonate using carbon dioxide and ethylene oxide as monomers and a non-phosgene polycarbonate process using wire reactors in melt polymerization of bisphenol A and diphenyl carbonate and widens the non-phosgene polycarbonate process in the world.
Other non-phosgene polycarbonate preparation techniques attempted by various companies include polycarbonate preparation techniques using melt polymerization and solid state polymerization. In particular, solid state polymerization is the best process to overcome preparation of polycarbonate with a high molecular weight, as one drawback of the non-phosgene process and a variety of techniques to realize this preparation are being introduced.
Solid state polymerization of polycarbonate requires induction of crystallization as a preliminary step. This induction step is considered the primary step to be solved in conventional techniques in order to realize solid state polymerization of polycarbonate.
However, in conventional techniques, crystallization of polycarbonate is induced in a solvent by introducing an organic solvent to dissolve polycarbonate, or by pelleting or grinding polymerized polycarbonate and crystallizing in the presence of heat or gaseous solvents, or by spraying polycarbonate dissolved in a solvent. These crystallization processes have disadvantages of use of a great amount of organic solvents and consumption of long time, thus making practical commercialization for solid state polymerization impossible.
In accordance with methods for crystallizing aromatic polycarbonate disclosed in U.S. Pat. No. 4,948,871 and JP Patent 2,546,724, crystalline porous polycarbonate is obtained by treating amorphous polycarbonate pre-polymers with acetone or heating the same for a long period of time. Such a crystallization method requires an additional process which cannot be continuously performed with melt polymerization and additional equipment and treatment time such as acetone recovery and washing. In addition, with this method, it is difficult to prepare porous polycarbonate particles with a uniform size and is thus impossible to realize polymerization uniformity in the solid state polymerization, the subsequent process. Another problem of this method is production of a great amount of non-uniform polycarbonate, in other words dust, due to friction and collision between particles caused by solid flow during solid state polymerization. This uneven polycarbonate dust is known to be a main factor degrading qualities of final products, such as induction of black spots and non-molten materials in the process of plastic molding.
In addition, U.S. Pat. No. 5,717,056 suggests a method for increasing polycarbonate crystallinity by treating a polycarbonate prepolymer with a dihydroxy aromatic compound and stirring the same in the presence of an alkali metal hydroxide catalyst and introduces realization of solid state polymerization of polycarbonate using the method. However, this method also disadvantageously requires stirring and reaction processes separately from melt polymerization to prepare a prepolymer and further entails a post-treatment process for solid state polymerization.
In accordance with the polycarbonate crystallization method introduced in U.S. Pat. No. 6,031,062, a polycarbonate prepolymer and diaryl carbonate are coextruded and then blended in an apparatus such as a mixer for a long time. Then, solid state polymerization is carried out in the mixer. This method has a disadvantage of impossibility of mass-production due to such a multi-step process.
U.S. Pat. No. 7,148,312 and KR Patent No. 0536528 disclose spray crystallization, comprising dissolving amorphous polycarbonate in a solvent to prepare a polycarbonate solution and spraying the solution using a nozzle to bring the solution into contact with a hot gas and thereby prepare crystalline polycarbonate. This crystallization method has an advantage of crystallization of polycarbonate within a relatively short time, but further requires a step for dissolving polycarbonate in a separate solvent after melt polymerization as a pre-crystallization step. Accordingly, this method also has a disadvantage of impossibility of efficient successive polymerization of non-phosgene polycarbonate. In addition, with this method, it is difficult to control particles of sprayed polycarbonate and thus solve the problems associated with polymerization uniformity in solid state polymerization and production of uneven polycarbonate dust of final products.
Accordingly, in order to solve these problems, there is an increasing need for a continuous non-phosgene polycarbonate process, based on efficient and simple crystallization process.